Heat oven to 400℉
Heat large stockpot over medium.
Season chicken pieces with first amount of salt and pepper.
Add oil to pan and brown chicken on all sides.
Remove chicken legs and reserve for addition later.
Add carrot, onion, and celery to pot, increasing heat to medium high, cooking while stirring for 3 minutes.
Add the garlic, thyme, and rosemary to the pot, cooking for a couple of minutes.
Add the chicken legs back to the pot, followed by the water.
Bring liquid to a boil over high heat.
Lower heat to medium low, simmering for 40 minutes.
In the saucepan, fill with potatoes and cover with water.
Add the salt to the water and bring to a boil over high heat.
Once the potatoes are tender, roughly 20 minutes, drain through a colander.
Return the potatoes to the saucepan, and combine with milk and butter.
Using the potato masher, smash until smooth and fluffy.
Cover and set aside while finishing the chicken filling.
Using tongs, remove the chicken legs and transfer to the refrigerator to cool to room temperature.
Add milk to the broth in the stockpot, increasing heat to high.
Add cold water to the cornstarch to form a slurry.
Add the hot sauce and Worcestershire.
Once the broth is boiling, add the slurry to the liquid while stirring in order to thicken, and then reduce the heat to a simmer.
Pick the chicken meat from the bones into small pieces.
Add the chicken to the sauce, stirring to combine.
Add the peas to the sauce, and stir through.
Pour the finished chicken filling into the casserole pan.
Using an offset spatula, form an even layer on top of the chicken with the mashed potatoes.
Melt the remaining butter.
Using a fork, create grooves in the mashed potatoes.
Evenly spoon the melted butter over the top of the mashed potatoes.
Place the casserole dish in the oven and bake until the top is browned, roughly 15 minutes.
